A digital micromirror device is known as a reflective image display element mounted on a projector. The digital micromirror device includes an image display surface formed with a plurality of microscopic micromirrors. On the image display surface, the digital micromirror device controls an inclination of each mirror surface to modulate an intensity of illumination light, thereby forming an image. That is, ON/OFF of each pixel of the digital micromirror device is indicated, for example, by turning the mirror surface at ±12° around a rotation axis forming an angle of 45° with respect to each side of the image display surface (that is, by driving the micromirrors with respect to one axis). In regards to the movement of the micromirrors, a new operation type digital micromirror device that drives the micromirrors with respect to two axes orthogonal to each other (Tilt & Roll Pixel DMD) is also proposed in Non Patent Literature 1.
In the reflective image display element such as the digital micromirror device, unnecessary light called OFF light which is not used in a projected image is generated. The OFF light becomes stray light and causes a contrast reduction of the projected image, or causes a temperature rise of the optical unit including a mechanical component. Therefore, in a projector using the digital micromirror device, it is important to process the OFF light incident on a prism and thus, various countermeasures have been proposed. For example, in Patent Literature 1, there has been proposed a configuration of causing a heat releasing body to absorb the OFF light. In addition, in Patent Literature 2, there has been proposed a configuration of separating the OFF light from projected light by increasing a height of a synthesizing prism to secure an optical path of the OFF light.